Process and device for ion thinning in a high resolution transmission electron microscope

ABSTRACT

The invention concerns an “in situ” ion-etching device for local thinning of a sample in a transmission electron microscope ( 1 ) with simultaneous electron microscopic observation. Towards this end, an ion beam device ( 2 ) is arranged in such a way that the finest possible ion probe is produced at the sample location and can be scanned over the sample surface. The ion beam ( 16 ) and sample ( 10 ) thereby enclose the flattest possible angle. To compensate for the magnetic field of the objective lens ( 5 ), the ion beam ( 16 ) is defected along a curved path onto the sample ( 10 ). In a preferred embodiment, an electrostatic cylinder capacitor sector field effects double focusing. The ion probe can be positioned, via the scanned ion image, onto a selected region of the sample by the secondary electrons ( 22 ) released from the sample ( 10 ). The sample location can be observed during the ion thinning process in electron transmission or electron diffraction. It is thereby possible to carry out target preparations under high-resolution observing conditions and to eliminate contaminant or reactive layers.

BACKGROUND OF THE INVENTION

The invention concerns a method and associated device for ion thinningof a sample in a sample region of a transmission electron microscopehaving an objective lens and comprising an ion source for the productionof an ion beam, an ion lens, a secondary electron detector for theproduction of an ion scan secondary electron image of the sample surfacewith the assistance of the secondary electrons released during scannedion irradiation and for the positioning of the ion beam, via the ionscan secondary electron image, onto a particular location of the sample.

Samples are prepared for high resolution transmission electronmicroscopy by being pre-thinned in a conventional manner, usingmechanical or chemical procedures, to an initial thickness of ca. 10 μm.The samples are subsequently ion thinned through bombardment with an ionbeam with the ion beam at as flat an angle as possible with respect tothe sample surface until a small hole of approximately 100-200 μm indiameter is produced in the middle of the sample. The sample is thensufficiently thin within a wedge-shaped region in the edge portion ofthe hole and is transparent for fast electrons in excess of 100 keV.

For high resolution transmission electron microscopy the amount ofinelastic scattering of the electrons in the sample is low, so that itis necessary for the sample to be sufficiently thin. A high lateralresolution of approximately 1.5 nm, corresponding to anelectromicroscopic enlargement of 100,000, can be achieved beginningwith a thickness of approximately 100 nm. In highest resolutiontransmission electron microscopy with which lateral resolutions of up to0.15 nm are achieved, corresponding to magnification factors of 1million, it is necessary for the sample to be thinned to a thickness ofless than 10 nm. In addition to high resolution electron microscopy,very thin object locations free from reactive layers, contaminationlayers and amorphous layers damaged by ion thinning, are also requiredin electron holography.

The usual ion thinning procedures utilized for sample preparation intransmission electron microscopy work in a substantially “blind” manner.Whether or not a desired object feature is properly enhanced with goodquality through preparation is more or less a question of luck. Devicesof prior art have therefore been proposed with which a sample in atransmission electron microscope sample holder can be thinned in anexternal etching device under simultaneous observation.

The publication Gatan Inc., 6678 Owens Drive, Pleasanton, Calif. 94566USA product specification “Precision Ion Milling System” (PIMS), model645, June 1987, has proposed imaging of the released secondary electronsfor this purpose. The publications H. Bach, Bosch Technische Berichte 1,1964, 10-13 and F. Nagata et al., Proc. 41 st. Confer. JSEM. 1985, 133,have proposed observing the sample for ion thinning in the first imagingplane of a transmission electron microscope in transmission mode.

These conventional etching devices have the disadvantage that theresolution which can be achieved for evaluation of the sample quality isinsufficient and that, after ion thinning, it is necessary to introducethe sample along with the sample holder into the field of the objectivelens for high resolution observation of the sample. The thin samplethereby reacts with air and with residual gas causing undesirablereaction and contamination layers.

It is furthermore disadvantageous that a plurality of transfers arenormally necessary between high resolution observation and additionalthinning in order to achieve the desired layer thickness. It is therebynot only difficult to once more locate the object position for renewedobservation, which can be extremely difficult for high lateralresolution, but the high resolution transmission electron microscopicobservation also causes disturbing contaminating layers due to the highenergy electron irradiation, in particular from hydrocarbons. It cantherefore be necessary to prepare a new sample location through renewedthinning at another location.

Another problem which occurs during high resolution transmissionelectron microscopy is associated with the fact that the objective lenscannot be arbitrarily switched-on and off. The objective lens producesthe magnetic field necessary for high resolution at the location of thesample of ca. 1 to 2 T and effects a constant magnification of 100 to200. When switching-on the objective lens, drifts occur due to variousinfluences such as current stability, heat expansion and heatequilibration effects as well as other causes which require a timeduration of two hours or longer to damp to a level sufficiently stablefor observation. It is therefore not possible to quickly change fromhigh resolution observation with switched-on objective lens having amagnification of approximately 1,000,000 to low, long focal lengthmagnification with the objective lens switched-off with, if appropriate,switching-on a mini or intermediate lens producing no magnetic field inthe objective plane having ca. 10,000-fold magnification. On the otherhand, due to the very limited amount of space particularly in the regionof the objective lens, it is not possible to move a sample in the vacuumof the transmission electron microscope out of the first imaging planeposition into the objective plane. In order to do this, the sample mustbe passed through a vacuum lock.

DE 29507225 U1 proposes an ion beam preparation device for electronmicroscopy which facilitates ion thinning during simultaneous electronmicroscopic observation. A high resolution observation is however notpossible with the configuration proposed therein, since no magneticfield is present at the location of the sample due to the lack of anobjective lens located therein. The electron energy is also low. Thesample is not disposed in the sample region of the objective lens of atransmission electron microscope.

JP-(A) 6-231719 proposes a preparation method for the preparation ofcuts in semiconductors with which a sample location having a passiveprotective layer of ca. 100 nm thickness is thinned in the objectiveplane of the objective lens of a transmission electron microscope usinga fast-ion-beam-ion source of 30 keV. The high energy of the ionssubstantially reduces problems with regard to working separations butcauses radiation damage and the large amount of material removal leadsto deposits on the lenses and therefore to imaging distortions. Nor isit possible with the apparatus proposed in this publication to thin thesample under high resolution conditions, i.e. with magnifications inexcess of 10,000 or 100,000, since the ion beam is not incident on thesample when the objective lens is switchedon. The publication thereforeproposes switching back and forth between ion thinning and higherresolution which is associated with the above mentioned disadvantages.

Those of average skill in the art have believed that it is necessary tocarry out ion thinning with the objective lens switched-off to allow theion beam to be incident on the sample location. The disadvantagesassociated therewith were accepted up to this point in time.

SUMMARY OF THE INVENTION

Departing from this prior art it is the underlying purpose of theinvention to improve the preparation techniques for transmissionelectron microscopy in such a manner that the sample can be thinned “insitu” under simultaneous high resolution transmission electromicroscopicobservation in the objective plane of the transmission electronmicroscope to prevent renewed reformation of reactive layers in air, toshorten the duration of the experimental times, and to be able to thinparticular sample locations or sample regions to a defined monitoredextent.

This purpose is achieved in accordance with the invention with anion-etching device for ion thinning of a sample in a sample region of atransmission electron microscope with an objective lens, including anion source for the production of an ion beam and an ion lens, asecondary electron detector for the production of an ion scanningsecondary electron image of the sample surface with the assistance ofthe secondary electrons released during scanned ion irradiation and forthe positioning the ion beam, via the ion scanned secondary electronimage, onto a particular sample location. The invention is characterizedin that the incidence direction of the ion beam into the magnetic sectorfield of the objective lens through which the ions pass is chosen insuch a fashion that the magnetic sector field passed through by the ionsdeflects the ions along a curved path onto the sample so that, with theobjective lens switched-on, the sample location can be simultaneouslyobserved along with the ion scanned secondary electron image and withthe electron beam of the transmission electron microscope intransmission mode.

Within the framework of the present invention, it has surprisingly beenfound th at the extremely difficult requirements associated withtransmission electron microscopes due to the high magnetic field and thevery limited amount of space caused by the objective lens can besatisfied by compensating for the influence of the magnetic field of theswitched-on objective lens on the ion beam by introducing the ion beamto the sample location under the influence of the magnetic field alongan ion optical path which is not a straight line rather curved without,as had been thought necessary up to this point in time, having toswitch-off the objective lens for ion thinning under simultaneoustransmission electron microscopic observation. Towards this end, the ionbeam device is disposed in a mechanically decentralized manner, thatmeans by a skewed introduction of the ion beam, or the ion beam passesspecial deflection plates effecting a slanted introduction or deflectionof the beam. In accordance with the invention, the ion beam is preciselydeflected or curved to once more be incident on the sample under theinfluence of the magnetic field perpendicular to the objective plane.

The invention achieves goals which those of average skill in the arthave been striving to achieve for some time. The invention allows, forthe first time and under high resolution conditions i.e. withmagnification factors in excess of 10,000 to ca. 1,000,000 correspondingto lateral resolutions of 1.5 nm to approximately 0.15 nm, for the localthinning of a sample location or a sample region “in situ” to therebyprepare desired sample locations and sample thickness without having tointroduce the sample into an external etching device or into the firstimaging plane of the transmission electron microscope or withoutrequiring a long lead time for the achievement of a stable operationcondition of the objective lens. The controlled ion thinning furthermorereduces soiling problems in the transmission electron microscope causedby preparation.

In accordance with an additional advantageous feature, an ion focus on asample location can be adjusted by means of the ion lens. The ion focusadvantageously has a diameter between 0.5 and 100 μm, preferentiallybetween 0.5 and 20 μm and particularly preferentially between 1 and 10μm. It thereby corresponds to the lateral dimensions of an observedobject location so that a local ion thinning can be carried out which isnot spreadout over a certain area. Deposits on the objective lens and inother regions of the transmission electron microscope with theassociated soiling problems are thereby substantially reduced.

In accordance with prior art, an ion focus of approximately 0.1 μmdiameter can be achieved with ion sources operating under optimalconditions. This is however not possible in accordance with presentconventional techniques for transmission electron microscopy due to thedifficult conditions caused by the dimensions and the magnetic field ofthe objective lens. In order to achieve as small a beam diameter aspossible at the sample location it is, however, advantageous for ionoptical reasons to maintain as small a working separation between theion beam device and the sample location as possible, preferentially lessthan 5 cm, since the imaging errors of the ion lens increase withincreasing focal length.

In accordance with another advantageous feature, the ion energy is lessthan 5 keV, preferentially less than 3 keV in order to reduce radiationdamage in the sample. Although focusing is thereby more difficult thanat higher ion energies, it has turned out that even low energies can beutilized within the framework of the invention. The ion source can, forexample, be a gas ion source using cold ionization, for example, asaddle field ion source or have electron impact ionization, for example,a hot cathode ion source which can be operated with rare gases (forexample Ar or He) or with reactive gases (for example O₂, N₂ or Freon).Gas-field ion sources (He, H₂) or liquid metal-field ion sources (forexample Ga or In), have turned out to be particularly advantageous forachieving as small a beam diameter as possible at the sample location,since same have lower energy and directional defocusing.

The curved ion optical path of the ion beam in accordance with theinvention while passing through the magnetic sector field of theswitched-on objective lens results in deflection of the ions onto thedesired sample location. By taking into consideration the deflection ofthe ions in the magnetic field and by adjusting an appropriate inputdirection, for example with slanted configuration of the ion beam sourceor by means of a curved pair (for example cylindrical capacitor) ofelectrostatic deflectors, it is possible, in most cases, to achievesufficient focusing of the ion beam onto the sample location even whenthe objective lens is switched-on, particularly when utilizingliquid-metal ion sources. A small amount of fanning-out and defocusingof the ion beam caused by the energy and directional defocusing of theion beam, which is tolerable in many cases, can not thereby becompletely avoided, even in the event of a homogenous magnetic field.

In order to further improve the focusing, a preferred feature proposesproviding deflection plates of an electrostatic cylindrical capacitorsector field configured in such a fashion that, in combination with themagnetic sector field of the objective lens through which the ions pass,double focusing of the ions with respect to energy and directionaldispersion is effected. The energy and directional dispersion of theions in the magnetic field can be compensated for by the cylindricalcapacitor sector field to facilitate sharper focusing.

Double focusing with respect to energy as well as initial direction ofthe ions is known in the art of mass spectrometers. For details one isreferred to the respective literature, e.g. A. Benninghoven et al.,Secondary Ion Mass Spectrometry, John Wiley & sons (1987) chapter 4.1.9.The double focusing provides a bunching of the ions entering into themagnetic field in parallel planes. Both directional focusing as well asvelocity focusing are thereby achieved. Particles of the same mass,whose velocities or directions deviate somewhat with respect to eachother, are approximately brought together at a point or line. Alsowithin the context of the invention with transmission electronmicroscopes, the lines can be focused to a point through additionalelectron optical components, for example stigmators.

In accordance with the invention, transmission electron microscopesamples can be locally ion thinned “in situ” directly in thetransmission electron microscope under high resolution observation. Inthis manner, target preparations can be carried out in a monitoredfashion and contamination and reactive layers can be directly removed inthe microscope vacuum.

Additional advantageous features and characteristics can be derived fromthe following embodiments of the invention which are described andexplained with regard to the schematic representation of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 show a transmission electron microscope according to theinvention,

FIG. 2 shows a detail of FIG. 1,

FIG. 3 shows a schematic sketch of an ion beam path with the objectivelens switched-off,

FIG. 4 shows a first path in accordance with the invention of an ionbeam with the objective lens switched-on.

FIG. 5 shows a second path of an ion beam in accordance with theinvention with the objective lens switched-on,

FIG. 6 shows a third path of an ion beam in accordance with theinvention with the objective lens switched-on,

FIG. 7 shows double focusing in accordance with Mattauch and Herzog,

FIG. 8 shows a cut through a pole piece system of a converging singlefield objective lens with the magnetic field dependence and sample, and

FIG. 9 shows a sketch of the locating of the sample in a sample region.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a transmission electron microscope 1 having an ion-etchingdevice in accordance with the invention and electronic control of theion beam device 2. The electron optical column of a transmissionelectron microscope 1 typically consists essentially of an electron beamsource 3, the condenser lenses 4, the objective lens 5, the projectionlenses 6 and the observation region 7 with a fluorescent screen in theobservation plane 8. The electron beam generator 3 produces electronshaving energies in excess of 100 keV and normally of 200 keV. FIG. 1also shows the objective plane 9, in which the sample 10 is located insample region 11 and the first imaging plane 12 in which the electronimage of the objective lens 5 is located. The sample 10 is located inthe objective plane 9 between the pole pieces 13 of the objective lens 5during imaging.

The ion beam device 2 includes an ion source 14 and an ion lens 15 forthe production of an ion beam 16. The ion beam device is disposed in theobjective plane 9 at the same level as the sample 10 during imaging. Theion beam device is connected to the microscope column vacuum with aspecial vacuum flange in such a fashion that a pressure step is formedat the sample location relative to the vacuum region in the objectivelens 5 via of a collimator 17 built into the ion beam device 2. Thevacuum region in the ion beam device 2 between the collimator 17 and anadditional collimator 18 can be evacuated via a second pump, independentof the objective or sample region 11 of the transmission electronmicroscope 1. This differential pumping system prevents the vacuum ofthe ion beam device 2 from influencing the sample region 10 and viceversa.

The electronic control of the ion beam device 2 includes a voltagesupply 19 for the ion source 14 and for the ion lens 15, a scangenerator 20 and a deflection amplifier 21. The ion beam 16 can bescanned over the sample 10 using the scan generator 20. A rasterscanning ion image can be displayed using the secondary electrondetector 23 on the scanning ion image monitor 24 via the secondaryelectrons 22 thereby released from the sample 10 (the arrow illustratesthe path of the secondary electrons 22). The scanning electron imageproduced by the electron beam of the transmission electron microscope 1in reflection or transmission can simultaneously be observed on thescanning electron image monitor 25. In this fashion, the ion beam 16 canbe easily aligned onto a particular sample location 26 or a particularsample region of the sample 10 at which ion thinning should be effectedusing the ion beam 16. The sample location 26 can be observed in theobservation plane 8 with the transmission electron microscope 1 duringsimultaneous ion thinning.

The ion beam 16 can be guided by means of control deflection plates in ascan-like fashion over the sample surface. The transmission electronmicroscope 1 can preferentially be a scanning transmission electronmicroscope, wherein the deflection of the ion beam 16 is advantageouslycontrollable by the scanning unit of the scanning transmission electronmicroscope and the secondary electron image can be recorded by means ofthe secondary electron detector 23 of the scanning transmission electronmicroscope.

FIG. 2 shows the transmission electron microscope 1 of FIG. 1 and theion beam device 2 without the electronic control. A long focal lengthmini lens (not shown) may be disposed in the vicinity of the projectionlens 6 to effect a magnification of ca. 10,000 when the objective lens 5is switched-off. One recognizes the very limited amount of space in thevicinity of the pole pieces 13 of the objective lens 5 which makes theholding of the sample 10, the introduction of the ion beam 16, as wellas the detection of the secondary electrons 22 using the secondaryelectron detector 23 difficult and which prevents displacement of thesample 10 within the transmission electron microscope 1. into the firstimaging plane 12.

FIG. 3 shows the path of the ion beam 16 when the magnetic field 27 ofobjective lens 5 is switched-off. The ion lens 15 focuses the ion beam16 onto the sample 10. The collimators 17, 18 form a pressure step toseparate the vacuum region of the ion beam device 2 from sample region11. The ion beam 16 can be scanned over the sample 10 in the objectiveplane 9 and at right angles thereto via the deflection plates 28 andsteered onto a particular desired sample location 26. Only smallmagnifications with reduced resolution are thereby possible throughmagnification via the projective lens 6 or another auxiliary lens.

When the magnetic field 27 of the objective lens 5 is switched-on, theion beam 16 is also deflected beginning at the pole piece edge 29 of theobjective lens 5, by the strong magnetic field 27 of the objective lens5 and is no longer incident on the sample 10. This cannot be compensatedfor by an axis-parallel deflection plate 28. Ion-etching is thereby onlypossible in the conventional configuration shown when the objective lens5 is switched-off. If the sample 10 is to be observed with highresolution and magnification, one must switch back and forth betweenion-etching and transmission electronmicroscopic observation, whereinthe above mentioned disadvantages with regard to reactions on the sample10 and the necessary experimental time must be accepted.

FIG. 4 shows a first configuration in accordance with the invention withwhich the incident direction of the ion beam 16 into the magnetic field27 though which the ions pass is chosen in such a fashion as to deflectthe ion along a curved path leading, when the objective lens 5 isswitchedon, onto the sample 10 and in particular onto a desired locationof the sample 26. The ion beam device 2 is shown in a schematic fashion.The ion beam 16 is not introduced radially into the magnetic field 27,rather at a particular incident angle. Towards this end, the ion beamdevice 2 is displaced sideward or rotated.

FIG. 5 shows a configuration alternate to that of FIG. 4 in which theion beam 16 is initially directed radially onto the sample 10 but, priorto entrance into the magnetic field 27, is deflected by means of a pairof electrostatic deflection plates having curved deflection plates 28 insuch a fashion that the ions are incident on the sample 10 along acurved path. The displacement of the ion beam 2 in accordance with FIG.4 can also be combined with the electrostatic deflection in accordancewith FIG. 5.

When the ion beam 16 has sufficient sharpness with respect to its energyand direction, it is possible, with the configurations in accordancewith FIG. 4 and 5, to achieve a satisfactory focus on the sample 10.However the energy or directional dispersion in the magnetic field 27increases with increasing energy defocusing or directional defocusing ofthe ion beam 16 leading to a fanning-out of the ion beam 16 at thesample location 26. In this case, double focusing (FIG. 6), isadvantageous in order to compensate for the energy and directionaldispersion. The ion-optic geometry and the field strength of theelectric cylinder capacitor sector field 30 are thereby chosen in such amanner that, together with the geometry and the field strength of themagnetic sector field 27 of the objective lens 5, a double focusingcondition is effected at the sample 10 (see also A. Benninghoven et. alSecondary Ion Mass Spectrometry, John Wiley & Sons, (1987)).

The initial ion beam 16 can thereby be directed radially or non-radiallyonto the sample 10. The deflection plates 28 form segments of acylindrical capacitor 30. In this manner the ion beam 16 is not onlyguided along a curved path to be deflected onto the sample 10 under theinfluence of the magnetic field 27 but, by means of the double focussingof the cylindrical capacitor 30 in combination with the magnetic field27, the energy and directional dispersion of the ion beam 16 is alsocompensated for to achieve a sharper focus on the sample 10.

FIG. 7 shows a particular advantageous embodiment of double focusingwhich is referred to in the literature (see for example A. Benninghovenet al.) with regard to mass spectrometers as the configuration inaccordance with Mattauch and Herzog. This is a special configurationwith which the sector angle of the cylindrical capacitor 30 is 31.8° andthe incident and exit angle of the ion beam 16 in the cylindricalcapacitor is 90°. The ion path is shown for two differing ion massesinitiating from a point P1 with energy and directional dispersion. Onenotices that, despite their energy and directional dispersion, ions of afirst mass are focussed at a first focus 31 and ions with a second massat a second focus 32. Of course, in the context of the invention,isotopes of differing masses are not used or at least substantiallyisotope-pure ion sources 14 so that the desired mass separation into twodiffering focuses 31, 32 needed in mass spectrometers is irrelevant.However, the compensation of the energy and directional dispersion canbe advantageously utilized. With other magnetic field 27 configurationsthan the ones shown having a particular electrostatic field of acylindrical capacitor 30, it is possible, as shown in the literature, toalways select a cylindrical capacitor 30 which, in combination with themagnetic field 27, effects double focusing.

FIG. 8 shows a schematic sketch of a cut through the objective lens 5 orits pole pieces 13 of a transmission electron microscope 1. The lens isa condenser single field lens as utilized in prior art for effectinghigh resolution. Large magnification can be achieved by means ofsuitable projection lenses 6. However, for high-resolution, as strong amagnetic field as possible is desirable at the location of the sample 10in particular more than 0.5 T and preferentially at least 1 to 2 T. Acondenser single field objective lens is a lens with which the sample 10is disposed between two pole pieces 13. A condenser single field lens isboth a condenser and an imaging lens.

In order to be able to achieve a high magnetic field 27 at the locationof the sample 10, the sample region 11 is very confined. The passage forthe electron beam 13 has, depending on the type and resolution of thetransmission electron microscope 1, a typical diameter of 0.5 to 1 cmand the separation with respect to the pole pieces 13 in the directionof the electron beam 33 is ca. 0.5 to 1 cm. The ion beam 16advantageously travels, in the region of the objective lens 5, in thesample or objective plane 9 of the objective lens 5. It is, however, inprinciple also possible for the ion beam 16 to travel in a plane whichis at an angle other than a right angle with respect to the electronbeam 33. In this event, the increased influence of the fringe field 34on the ion beam 16 must be taken into account or a theoretical orexperimental determination of a suitable curved path as well asassociated deflection plates 28 or the associated cylindrical capacitor30 are required, thereby increasing complications.

FIG. 9 shows a schematic configuration of the sample 10 duringirradiation with an electron beam 33 and the ion beam 16. The sample 10is held by a sample holder (not shown) preferentially in a goniometersuspension configured in such a fashion that the sample surface can betilted at an angle between −10° to +10° with respect to the ion beam 16under simultaneous ion thinning and transmission electron microscopicobservation. The preceding treatment normally results in a hole of ca.100 to 200 μm diameter in the sample 10. The edge of this hole iswedge-shaped and represents the usable sample region. Subsequentprocessing of the sample 10 to a desired sample thickness at a desiredlocal sample location 26 can be carried out at the edge by means ofgrazing incidence of ion beam 16, wherein the sample location 26 can besimultaneously observed during ion thinning at high resolution with theelectron beam 33.

List of Reference Symbols

1 transmission electron microscope

2 ion beam device

3 electron beam source

4 condenser lens

5 objective lens

6 projection lens

7 observation region

8 observation plane

9 objective plane

10 sample

11 sample region

12 first imaging plane

13 pole piece

14 ion source

15 ion lens

16 ion beam

17 collimator

18 additional collimator

19 voltage supply

20 scan generator

21 deflection amplifier

22 secondary electrons

23 secondary electron detector

24 ion scan image monitor

25 electron scan image monitor

26 sample location

27 magnetic field

28 deflection plates

29 pole piece edge

30 cylindrical capacitor

31 focus

32 focus

33 electron beam

34 fringe field

We claim:
 1. An ion-etching device for ion thinning of a sample in asample region of a transmission electron microscope having an objectivelens and comprising an ion source for the production of an ion beam andan ion lens, a secondary electron detector for the production of an ionscan secondary electron image of the sample surface with the assistanceof secondary electrons released by scanning ion irradiation and forpositioning the ion beam via the ion scan secondary electron image ontoa particular sample location, such that when the objective lens isswitched-on the sample location can be observed simultaneously with theion scan secondary electron image and with the electron beam of thetransmission electron microscope in transmission wherein the ion energyis less than 5 keV and the ion beam in the vicinity of the objectivelens travels in the objective plane of the objective lens and wherein,by taking into consideration the deflection of the ions of the magneticfield of the objective lens, the introduction of the ion beam into amagnetic sector field of the objective lens through which the ions passis chosen in such a fashion that for the compensation of the influenceof the magnetic field of the objective lens the magnetic sector fieldthrough which the ions pass deflects the ions along a curved path ontothe particular sample location in a defined manner.
 2. The ion-etchingdevice according to claim 1, wherein an ion focus can be adjusted at theparticular sample location by the ion lens and the ion focus at theparticular sample location has a diameter between 0.5 and 100 μm.
 3. Theion-etching device according to claim 1, further comprising deflectionplates of an electrostatic cylindrical capacitor sector field which areconfigured, in combination with a magnetic sector field of the objectivelens through which the ions pass, to effect a double focusing of theions with regard to energy and initial direction.
 4. A transmissionelectron microscope having an objective lens and an ion-etching devicefor ion thinning of a sample comprising an ion source for the productionof an ion beam and an ion lens, a secondary electron detector for theproduction of an ion scan secondary electron image of the sample surfacewith the assistance of secondary electrons released by scanning ionirradiation and for positioning the ion beam via the ion scan secondaryelectron image onto a particular sample location, such that when theobjective lens is switched-on the sample location can be observedsimultaneously with the ion scan secondary electron image and with theelectron beam of the transmission electron microscope in transmission,wherein the ion energy is less than 5 keV and the ion beam in thevicinity of the objective lens travels in the objective plane of theobjective lens and wherein by taking into consideration the deflectionof the ions in the magnetic field of the objective lens, theintroduction of the ion beam into a magnetic sector field of theobjective lens through which the ions pass is chosen in such a fashionthat for the compensation of the influence of the magnetic field of theobjective lens, the magnetic sector field through which the ions passdeflects the ions along a curved path onto the particular samplelocation in a defined manner.
 5. The transmission electron microscopeaccording to claim 4, wherein an ion focus can be adjusted at theparticular sample location by the ion lens and the ion focus at theparticular sample location has a diameter between 0.5 and 100 μm.
 6. Thetransmission electron microscope according to claim 5, furthercomprising a sample holder for holding a sample surface, duringsimultaneous ion thinning and transmission electron microscopicobservation, at an angle of −10° to +10° relative to the ion beam. 7.The transmission electron microscope according to claim 4, furthercomprising deflection plates of an electrostatic cylindrical capacitorsector field which are configured, in combination with a magnetic sectorfield of the objective lens through which the ions pass, to effect adouble focusing of the ions with regard to energy and initial direction.8. The transmission electron microscope according to claim 4, having anobjective lens comprising a condenser single field lens.
 9. Thetransmission electron microscope according to claim 4, wherein themagnetic sector field at the sample location is greater than 0.5 T. 10.The transmission electron microscope according to claim 4, wherein theenergy of the electrons for production of the transmission electronimage is greater than 100 keV.
 11. The transmission electron microscopeaccording to claim 4, adapted for high resolution transmission electronmicroscopy having a transverse resolution better than 1.5 nm during asimultaneous ion thinning.
 12. A method for ion thinning of a sample ina sample region of a transmission electron microscope having anobjective lens comprising the steps of: producing an ion beam with anion source; producing a scanning electron secondary electron image ofthe sample surface with the assistance of the secondary electronsreleased during a scanned ion irradiation; positioning the electron beamvia the ion scanned secondary electron image onto a particular samplelocation such that when the objective lens is switched-on, the samplelocation is observed in transmission with the electron beam of thetransmission electron microscope simultaneously with the ion scansecondary electron image; wherein the ion beam travels in the region ofthe objective lens in the objective plane of the objective lens andwherein the ions of the ion beam enter into a magnetic sector field ofthe objective lens with an energy of less than 5 keV and wherein bytaking into consideration the deflection of the ions in the magneticfield of the objective lens the ions are introduced into the magneticsector field of the objective lens in such a fashion that for thecompensation of the influence of the magnetic field of the objectivelens the magnetic sector field through which the ions pass deflects theions along a curved path onto the particular sample location in adefined manner.
 13. The method according to claim 12, wherein an ionfocus at the sample location is produced by the ion lens.
 14. The methodclaim 12, wherein the ions travel through an electrostatic cylindricalcapacitor sector field which is configured, in combination with themagnetic sector field of the objective lens through which the ions pass,to effect a double focusing of the ions with regard to energy andinitial direction.